1. Field of the Invention
The present invention relates to a process for incorporating an ultraviolet (UV) light absorber into a polyester or copolyester composition. More particularly, the present invention relates to a process for incorporating a UV light absorber into a polyester prepared using direct esterification of a dicarboxylic acid and a diol.
2. Background of the Invention
Polyester is a polymeric resin widely used in a number of packaging and fiber based applications. Commercial polyester production, in general, involves direct esterification, where the desired glycol, in molar excess, is reacted with an aromatic dicarboxylic acid to form an ester; or by transesterification or ester exchange if the starting aromatic moiety is a low molecular weight diester of an aromatic dicarboxylic acid, such as dimethyl terephthalate (DMT) which is polycondensed under reduced pressure and at elevated temperatures form to poly(ethylene terephthalate) (PET). Since the product of these condensation reactions tend to be reversible and in order to increase the molecular weight of the polyesters, this reaction is often carried out in a multi-chamber polycondensation reaction system having several reaction chambers operating in series. In the case where the starting aromatic moiety is an aromatic dicarboxylic acid, water is the by-product of the reaction. In the case where the starting aromatic moiety is a diester of an aromatic dicarboxylic acid, such as DMT, methanol is the by-product of the reaction. In either case, the reaction by-product is removed by distillation.
The diglycol ester then passes to the second, prepolymerization step to form intermediate molecular weight oligomers before passing to the third, melt polyesterification step or polycondensation step operated at low pressure and high temperature. The molecular weight of the polymer chain continues to increase in this second chamber with volatile compounds being continually removed. This process is repeated successively for each reactor, with each successive reactor being operated at lower and lower pressures. The result of this step wise condensation is the formation of polyester with high molecular weight and a higher inherent viscosity relative to the esterification step. For some applications requiring yet higher melt viscosity, solid-state polymerization is practiced.
Poly(ethylene terephthalate) or a modified PET is the polymer of choice for making beverage and food containers such as plastic bottles and jars used for carbonated beverages, water, juices, foods, detergents, cosmetics, and other products. However, many of these products are deleteriously affected, i.e., degraded, by ultraviolet (UV) light at wavelengths in the range of approximately 250 to 390 nanometers (nm). It is well known that polymers can be rendered resistant to UV light degradation by physically blending in such polymers various UV light stabilizers such as benzophenones, benzotriazoles and resorcinol monobenzoates. Although these stabilizers function well to absorb radiation, many of these compounds would decompose under the conditions at which polyesters are manufactured or processed. Decomposition of such stabilizers frequently causes yellow discoloration of the polyester and results in the polyester containing little, if any, of the stabilizer.
U.S. Pat. No. 4,617,374 to Pruett et al. discloses the use of certain UV-absorbing methine compounds that may be incorporated into the polyester or a polycarbonate composition. These UV absorbing compounds have been found to be useful in the preparation of polyesters such as poly(ethylene terephthalate) and copolymers of poly(ethylene terephthalate) and poly(1,4-cyclohexylenedimethylene terephthalate). The compounds enhance ultraviolet or visible light absorption with a maximum absorbance within the range of from about 320 nm to about 380 nm. Functionally, these compounds contain an acid or ester group which condenses onto the polymer chain as a terminator. Pruett et al. teach preparing the polyester using transesterification, i.e., where DMT is a starting material, and adding the UV absorbing compound at the beginning of the process. However, it has been unexpectedly discovered that the process by which the polyester is prepared and the point at which the UV absorbing compound is add to the polyester contributes to the efficiency at which such UV absorbing compound(s) are incorporated into the polyester. It has been discovered that these UV absorbing compounds are not readily incorporated into the polyester prepared using direct esterification as described above. The loss of UV absorbing compounds results in added costs for the polyester formation.
U.S. Pat. Nos. 6,207,740 and 6,559,216 to Zhao et al. discloses certain polyoxyalkylenated methine-based compounds are suitable for use as UV absorbing compounds in thermoplastics. These patents teach that such polyoxyalkylenated methine-based compounds are normally liquid and may be blended with already polymerized themoplastics, such as polyesters, polyolefins, halogenated polymers, and polyamides. However, the patents do not disclose or provide a fair suggestion as to if or when such polyoxyalkylenated methine-based compounds could be incorporated into the polyester composition during the manufacture and particularly if the polyester is made utilizing direct esterification of a diacid and a diol.
Accordingly, there is a need for improved methods of incorporating UV absorbing compounds, into polyester compositions made using the method of direct esterification, and particularly UV absorbing compounds those of the type described in U.S. Pat. No. 4,617,374.